USES OF REFRACTION 115 



learned that the atmosphere varies in density from level to 

 level; hence all the light which travels through the atmos- 

 phere is constantly deviated from its original path, and before 

 the light reaches the eye it has undergone many changes in 

 direction. Now we learned in Section 102, that the direction 

 of the rays of light as they enter the eye determines the di- 

 rection in which an object is seen ; hence the sun, moon, 

 and stars seem to be along the lines which enter the eye, 

 although in reality they are not. 



109. Uses of Refraction. If it were not for refraction, or 

 the deviation of light in its passage from medium to medium, 

 the wonders and beauties of the magic lantern and the cam- 

 era would be unknown to us ; sun, moon, and stars could not 

 be made to yield up their distant secrets to us in photographs ; 

 the comfort and help of spectacles would be lacking, spec- 

 tacles which have helped unfold to many the rare beauties of 

 nature, such as a clear view of clouds and sunset, of humming 

 bee and flying bird. Books with their wealth of entertain- 

 ment and information would be sealed to a large part of man- 

 kind, if glasses did not assist weak eyes. 



By refraction the magnifying glass reveals objects hidden 

 because of their minuteness, and enlarges for our careful con- 

 templation objects otherwise barely visible. The watchmaker, 

 unassisted by the magnifying glass, could not detect the tiny 

 grains of dust or sand which clog the delicate wheels of our 

 watches. The merchant, with his lens, examines the separate 

 threads of woolen and silk fabrics to determine the strength 

 and value of the material. The physician, with his invaluable 

 microscope, counts the number of infinitesimal corpuscles in 

 the blood and bases his prescription on that count; he ex- 

 amines the sputum of a patient to determine whether tuber- 

 culosis wastes the system. The bacteriologist with the same 

 instrument scrutinizes the drinking water and learns whether 



